photoelastic

Photoelasticity is an optical technique used to visualize and quantify internal stresses in transparent materials by exploiting stress-induced birefringence. When an initially isotropic material is loaded, its refractive indices split along principal stress directions, causing phase retardation between the polarized light components passing through the specimen.

In a typical setup, polarized light passes through a loaded specimen and an analyzer is used to observe fringe patterns. Isochromatic fringes, which appear when the light colors shift with load, correspond to constant differences in principal stresses. The fringe order N is related to the stress difference by a material-dependent stress-optic coefficient, the specimen thickness, and the light path. Higher fringe density indicates larger stress differences.

Methods vary from two-dimensional to three-dimensional analyses. Two-dimensional photoelasticity uses a plane polariscope to view patterns in a flat section and is common for engineering components under plane stress. Circular or full-field polariscopes enable more comprehensive views, while dynamic photoelasticity employs high-speed imaging to capture transient events. Calibration and careful interpretation are essential, as fringe patterns depend on material properties, geometry, mounting, and loading.

Materials used are typically transparent polymers, resins, or glass that exhibit noticeable birefringence under stress. The technique is widely used in engineering design, failure analysis, and education to validate stress distributions in parts, joints, or composites. While powerful, photoelasticity requires clear access to the region of interest and appropriate calibration for quantitative results.